DE19600108C2 - Method and arrangement for operating a resistive moisture sensor - Google Patents

Description

The invention relates to a method and an arrangement for operating a resisti ven humidity sensor.

Such a method and an arrangement are known from DE 38 03 138 A1 known. Resistive moisture sensors are used, for example, Moisture or raindrops on the windshield of a motor vehicle report to control wipers automatically or semi-automatically. To are conductive on the windshield or other suitable location NEN applied so that the resistance between the conductor tracks with the Reduced the amount of moisture.

From DE 40 33 332 A1 a windshield wiper arrangement is known, which a moisture sensor to detect the presence of moisture and egg NEN vibration sensor to detect a caused by raindrops Has vibration, with a control unit corresponding to a wiper motor the output signals from the moisture sensor and the vibration sensor sor operated.

It does not matter whether the windshield needs to be wiped or washed only from the amount of moisture, but also from the composition of the Covering or the wetting on the windshield.

The object of the present invention is to provide a this composition differentiated measurement of moisture to allow on a disc.

This object is achieved in that briefly after a jump in a moisture sensor Signals and at least later the Conductivity of the moisture sensor is measured and that with the measured values information about the species and the Degree of contamination on the disc can be determined.

The method according to the invention enables differentiation between rewetting the pane with clean water and wetting with dirty or salty water, what, for example, by vehicles driving ahead is whirled up. There is also a Differentiation between large drops that contain clean water and smaller drops Dirty or salt water.

An advantageous embodiment of the invention The method consists in that the measured values as Input variable are fed to a table in which Output values are stored that are used to control a Window cleaning system can be used. So that's one advantageous control of a window cleaning system in Depends on the type of contamination of the pane possible.

Although as such within the scope of the method according to the invention after a voltage jump at any number of times the conductivity of the moisture sensor can be measured, it turned out to be favorable that shortly after the Jump of the signal a first measured value and to a later one At the time a second measured value arises that a switching signal  for a window cleaning system by comparing the second measured value is derived with a threshold value and that the threshold value of the first measured value, preferably the Difference between the first measurement and the second Measured value, is dependent. The can expediently first measured value delayed as far as the signal jump be that the influence of lead wires or the influence of capacitors to prevent voltage spikes be eliminated.

An advantageous embodiment of the invention The method is that the moisture sensor supplied electrical signal from rectangular Voltage pulses are formed and that the measured values by Measurements of the current through the moisture sensor when connected Tension can be gained. In this embodiment results a sensitivity of the moisture sensor, which in the is essentially independent of the degree of wetting. Takes For example, assume that all of the humidity sensor impinging drops have approximately the same conductance, this is how all drops work - from the first to, for example 10th or 20th - also about the same signal swing. Depending on Use case is a measurement of the Voltage drop across the energized sensor also possible.

Another requirement for moisture sensors is that electrolytic removal of the conductor tracks and corrosion on body parts should be avoided. This is in advantageously in a further development of this Embodiment achieved in that alternately positive and negative voltage pulses are supplied.

Although for measurements after one jump given time is relatively short, can be erratic No changes in the sensor signal during this time  be excluded. For example, during this Time a drop hit the moisture sensor, causing the conductivity is suddenly increased. The conductivity However, if a drop is removed or by a Wiping suddenly fall. To be contingent Avoiding incorrect measurements is a further development of the inventive method provided that at a within a given time after the jump non-continuous course of each measured sensor signal instead of the measured values information determined after a previous jump about the nature and degree of pollution be used.

Should even jump during several measuring times Changes to the signal take place, so are each last valid information on the type and degree of Pollution applied repeatedly. At the first valid Measurement afterwards can be used to control the wiper, however immediately valid information about the species and the Degree of contamination with the help of the updated measured values be determined.

There is an advantageous embodiment of this training in that more than two within the given time Measured values are recorded that it is checked whether the temporal sequence of the measured values and the temporal sequence of the Differences between the measured values are monotonic, and that if applicable, the respective measured values applied and not applicable, the information from the measured values after a previous jump.

It is preferably provided that in the tests Monotony effects of quantization errors and superimposed interference signals are disregarded.  

An advantageous arrangement for carrying out the The inventive method is that for measurement of the current is a measuring resistor with the humidity sensor in series is switched and that the falling across the measuring resistor Voltage via an amplifier to an analog signal input a microcomputer can be fed and that in the microcomputer a program for recording both measured values, for correction of the threshold value comparison and for the threshold value comparison is provided.

However, it is also possible to carry out an arrangement to design the inventive method such that the signal in the form of current pulses to the humidity sensor is supplied that the measured values from the on the moisture sensor falling voltage can be obtained and that the over the Voltage falling voltage across an amplifier can be fed to an analog signal input of a microcomputer is.

The invention permits numerous embodiments. A of which is schematic in the drawing based on several Figures shown and described below. It shows:

Fig. 1 is a block diagram of an arrangement according to the invention,

FIG. 2 time diagrams for explaining the dependence of the current through the moisture sensor from contamination,

Fig. 3 is a flowchart of an existing in a microcomputer in an arrangement for carrying out the process program,

Fig. 4 are timing diagrams of various occurring in the arrangement of Fig. 1 signals,

Fig. 5 is a schematic representation of a table with correction values, which are processed in the program of Fig. 3,

Fig. 6 shows time diagrams of sensor signals with changes in the amplitude within a pulse,

Fig. 7 is a timing diagram for the formation of four measured values and

Fig. 8 is a flowchart of another present in a microcomputer in an arrangement for carrying out the process program.

The moisture sensor 1 consists of conductor tracks which form two intermeshed electrodes 2 , 3 and are connected to an evaluation circuit via connections 4 , 5 . The evaluation circuit contains a microcomputer 6 , at the output 7 of which a signal characterizing the humidity can be removed. In addition to other elements, the microcomputer 6 contains an analog / digital converter 8 with an analog signal input 9 . Outputs 10 , 11 of the microcomputer 6 signals can be removed. A switchable amplifier 12 is provided for applying a pulse-shaped voltage to the moisture sensor 1 , the output of which can be switched to high impedance via a control input. This control input receives a signal EN from the output 11 of the microcomputer 6 , while the signal input of the amplifier is connected to the output 10 carrying a signal S. The output of the switchable amplifier 12 is connected to the terminal 4 of the moisture sensor 1 via a capacitor 13 .

To measure the current through the moisture sensor, the connection 5 is connected to ground potential via a measuring resistor 14 . A capacitor 20 is used to discharge voltage peaks. The voltage at terminal 5 therefore corresponds to the current-proportional voltage drop of measuring resistor 14 and is supplied via a resistor 15 to the non-inverting input of a differential amplifier 16 . The inverting input receives a part of the output voltage of the differential amplifier 16 via a voltage divider 17 , 18 . The degree of amplification is thus determined by the division ratio. The output voltage of the differential amplifier 16 is fed to the analog signal input 9 of the microcomputer 6 .

Fig. 2a shows the course of a rectangular voltage U applied to the moisture sensor 1 , Fig. 2b shows the course of the voltage across the measuring resistor 14 , thus also the current I through the moisture sensor in the event of wetting with salt water and Fig. 2c in the case of wetting with distilled water in an experiment without the condenser 20 ( Fig. 1). The absolute size of the measured values at t1 and t2 strongly depends on the amount of moisture applied or the number of drops. The difference between the measured values is, however, due to the salinity or the ion concentration.

The voltage jump takes place at time t0, while a first measurement M1 takes place at time t1. Like the capacitor 20 ( FIG. 1), the delay between t1 and t0 is intended to avoid the measurements being influenced by voltage peaks. Shortly before the end of the rectangular pulse, a further measurement M2 of the current I is carried out at the time t2. A comparison of FIGS. 2b and 2c shows that at time t1 when wetting with salt water the conductivity is considerably greater than when wetting with distilled water and that the conductivity at t2 corresponds approximately to that at t1.

The program shown in FIG. 3 is started at 21 and continued at 22 with the acquisition of the measured values M1 and M2 and a specification of the threshold value S. In a program part 23 , a factor F is determined from a table as a function of M2 and the difference between the measured values M1 and M2, and the threshold value S is tracked using the factor F.

At 24 , the measured value M2 is then compared with the tracked threshold value S. If M2 is less than S, the program is repeated at 22 . However, if M2 has reached or exceeded the size of S, a wiping process is triggered at 25 and the program is then repeated at 22 .

Fig. 4 shows the course of the signals S and EN, the voltage U₄ at terminal 4 of the current I₁₄ through the measuring resistor 14th During the duration of the pulse pairs shown in lines c and d, the switchable amplifier 12 is switched through by the signal EN (line b), so that during this time the signal s (line a) reaches the terminal 4 via the capacitor 12 . The DC voltage component is suppressed by the capacitor, so that there is a voltage jump between the pulses of different polarities. During the interval between the pairs of pulses, the switchable amplifier 11 has a high resistance, so that the voltage (line c) initially remains at 0, even if the signal S jumps from L to H. Only when the signal EN returns to the value H does the positive level reach the terminal 4 as a positive edge of the next pair of pulses.

Basically, the moisture sensor 1 has an increasing conductivity with the amount of moisture, so that the current I₁₄ increases with increasing amount of moisture. This is indicated in Fig. 4 by the fact that to the left of the interruption two pairs of pulses the current I₁₄ with a larger amount of moisture and to the right of the interruption a pair of pulses in the case of a smaller amount of moisture are shown.

FIG. 5 serves to explain a table stored in the microcomputer 6 ( FIG. 1), in which a factor F is stored for a pair of values M2 and (M1-M2). The factors F can be determined empirically by driving tests. The corrected threshold value can be obtained in a simple manner by multiplying a previously determined trigger difference by the factor F. Including other system variables, such as the drying time constant of the sensor signal since the last one, results in correction fields that allow the trigger difference to be adjusted even more precisely to the current rain situation enable.

Fig. 6 shows the course of the signal in the event that drops fall on the moisture sensor between the individual pulses, which is illustrated by arrows T +. If the drops fall between the pulses onto the moisture sensor, the conductivity increases from pulse to pulse, which is shown in an idealized manner in FIG. 6, again only the positive pulses being considered here. The amplitude of the measurement pulses increases, but the basic course of the signal does not change during the measurement pulses. In the illustration according to FIG. 6a, a drop falls on the moisture sensor during the last measurement pulse shown. This increases the signal suddenly.

In the illustration according to FIG. 6b An on-humidity sensor drops away during a pulse (T-) is. As a result of these abrupt changes, a comparison of two measured values, as described in connection with FIG. 2, gives a falsified result. In order to recognize the occurrence of such a change in the signal, provision is made to determine a plurality of measured values, for example four measured values M1 to M4 at times t1 to t4 according to FIG. 7.

An evaluation of these measured values is possible with the program shown in FIG. 8, which can replace the program according to FIG. 3. After starting at 21 , the measured values M1 to M4 are recorded at 32 . At 33 it is then checked whether the measured values are either the same or represent a monotonically falling sequence. If the condition M1 M2 M3 M4 is not fulfilled, the measured values M1 and M4 are set at 34 to be the same as those from the last valid measurement. If the condition is fulfilled, however, a check is made at 35 to determine whether the differences between the measured values M1 to M4 are monotonically falling. This is the case if the condition δM1.2 δM2.3 δM3.4 is fulfilled. The new measured values M1 and M4 are then stored at 36 . Otherwise, the old measured values are read out of the memory at 34 .

At 37 , S is updated with the measured values M1 and M4, as was described in connection with FIG. 3. This is followed by a branch 38 , in which it is checked whether the condition M4 S is fulfilled. If applicable, a wiping process 25 is triggered, as in the program according to FIG. 3.

Claims (10)

1. A method for operating a resistive moisture sensor, in particular on a windshield of a motor vehicle, characterized in that the conductivity of the moisture sensor is measured shortly after a jump of a signal supplied to the moisture sensor and at least at a later point in time, and that information about the The type and degree of contamination on the pane can be determined. 2. The method according to claim 1, characterized in that the measured values as the input variable of a table are supplied, in which output values are stored, used to control a window cleaning system will. 3. The method according to claim 1, characterized in that shortly after the signal jumps a first measured value and closes at a later point in time a second measured value arises that a switching signal for a window cleaning system Comparison of the second measured value with a threshold value  is derived and that the threshold value from the first Measured value, preferably the difference between the first Measured value and the second measured value. 4. The method according to any one of the preceding claims, characterized in that the supplied to the moisture sensor electrical signal from rectangular voltage pulses is formed and that the measured values by measurements of the current obtained by the moisture sensor when voltage is applied will. 5. The method according to claim 4, characterized in that alternating positive and negative voltage pulses are supplied will. 6. The method according to any one of the preceding claims, characterized in that at one within one given time after the jump continuous course of the measured Sensor signal instead of the measured values after a previous jump determined information about the Type and level of pollution continues to be used will. 7. The method according to claim 6, characterized in that more than two measured values within the specified time be included that it is checked whether the temporal Sequence of the measured values and the temporal sequence of the differences between the measured values are monotonic, and that if applicable, the respective measured values applied and not applicable, the information from the measured values after a previous jump. 8. The method according to claim 7, characterized in that when testing for monotony effects of Quantization errors and superimposed interference signals except  Stay considered. 9. Arrangement for performing the method according to one of the preceding claims, characterized in that for measuring the current, a measuring resistor ( 14 ) with the moisture sensor ( 1 ) is connected in series and that the voltage drop across the measuring resistor ( 14 ) via an amplifier ( 16 ) can be fed to an analog signal input ( 9 ) of a microcomputer ( 6 ) and that a program for recording the measured values, for correcting the threshold value comparison and for threshold value comparison is provided in the microcomputer. 10. Arrangement for performing the method according to claim 1, characterized in that the moisture sensor the signal is supplied in the form of current pulses that the measured values obtained from the voltage drop across the moisture sensor and that the falling over the moisture sensor Voltage via an amplifier to an analog signal input a microcomputer can be fed.